Live pagerank = Legit ? No way man. Just keep going trough your natural 3 months or more PR check-up routine. If you need a Firefox plugin, for SEO reasons i would warmly recommend Search Status. It has pagerank, all kinds of data like IBL's, indexed pages, wayback machine, Alexa rank and tools, the ability to activate a nofollow filter to visibly see where nofollow's are in a webpage etc. I use for a year now, and i would die without it.
I installed the live pagerank plugin a little while ago and I've been keeping an eye on it... It seems to pull up the same kind of info as the live pagerank checkers and page rank predictors. Like you said basically, Expertu, doesn't do a whole lot of good, but I guess it's mildly useful... I'm going to try the one you speak of though, thanks!